The preparation and lithiation of 1-halogenocyclopropenes

Abstract

Reaction of a range of 1,1,2-trihalogenocyclopropanes (halogen = bromine, chlorine) with methyl-lithium in ether at –90–20 °C leads to 1,2-dehalogenation to the corresponding 1-halogenocyclopropene. The halogenocyclopropenes are readily lithiated by lithium–halogen exchange with a second equivalent of methyl-lithium to give 1-lithiocyclopropenes, which are in turn trapped by electrophiles; an exception to this is compound (21; X = Cl), which leads to 3-methylbuta-1,2-dienylidene (31) by initial lithium–hydrogen exchange and loss of lithium chloride. The cyclopropenes (5; R = Me, X = Br or Cl) and (21; X = Br or Cl) decompose even at temperatures around ambient, leading either to enynes (6; R = Me) or halogenoalkynes (18; X = Br or Cl). A 12C-labelling study indicates that C-1 of the cyclopropene (21; X = Cl) becomes C-2 of the alkyne (18; X = Cl) on rearrangement.